In recent years, aluminum phosphate has been used as a binder for refractories an adhesive in various uses, a coating agent, a raw material for paints, a curing agent for alkaline substances such as water glass and silica sol (cf. Japanese Patent Application Laid-Open (JP-A) No. 011200/78), a raw material for special coating for steel plates (cf. Japanese Patent Application Laid-Open (JP-A) No. 2002-069657), an antioxidant for carbon materials, etc. and the demand therefor has been increasing year by year.
In recent years, public attention has been focused on the function of an aluminum phosphate solution as an antioxidant for carbon materials. As compared with other materials, carbon material has characteristics such as low thermal expansion coefficient, superior electric conductivity and high resistance to high temperature, thermal impact and chemicals, whereby it has been widely utilized in the fields of metallurgy, electricity, chemistry, nuclear reactor, etc. Improvements in properties and development of uses have been intensively conducted for carbon powder and carbon fiber as well as for carbon nano-tube, fullerene, etc. but there are still many problems to be solved. One of them is the degradation of carbon material caused by oxidation. As antioxidants for carbon material, aluminum compounds such as aluminum phosphate and salts of organic acids with aluminum have been proposed.
For example, a compound of aluminum monophosphate and colloidal silica is used to prevent the oxidation of graphite electrodes for arc-type electric furnaces (cf. Japanese Patent Application Laid-Open (JP-A) No. 2000-169845). Silicic materials, aluminum phosphate and a mixture thereof are used as antioxidants for graphite materials (cf. Japanese Patent Application Laid-Open (JP-A) No. 2001-192284). And an organic acid salt with aluminum is used as an antioxidant for carbon materials (cf. Japanese Patent Application Laid-Open (JP-A) No. 207484/86).
The mechanism by which aluminum phosphate serves as an antioxidant for carbon material has been presumed to be that, when a carbon material is impregnated or coated with an aluminum phosphate solution followed by drying and heating, a dehydrating condensation of aluminum phosphate proceeds on the surface or in the inner area of the carbon material to form a heat-resisting antioxidant coat which prevents a direct contact of oxygen in the air with the carbon material at high temperature. When an aluminum phosphate solution is quickly dried and heated, a non-uniform antioxidant coat may be formed due to the transfer of the component onto the surface or the surficial layer. Therefore, a non-efficient drying and heating for long time has been required.